In recent years, in consideration of environmental conservation, attention has been paid to a lead-free piezoelectric ceramic composition which contains no Pb; however, it has been known that a piezoelectric d constant of the lead-free piezoelectric ceramic composition is generally low as compared to that of a lead-based piezoelectric ceramic composition such as PZT (PbTiO3—PbZrO3).
Under circumstances as described above, since a (K,Na)NbO3-based piezoelectric ceramic composition has a relatively high piezoelectric d constant (piezoelectric strain constant) among lead-free piezoelectric ceramic compositions, research and development of the (K,Na)NbO3-based piezoelectric ceramic composition has been aggressively carried out.
For example, in Patent Document 1, a piezoelectric ceramic composition has been disclosed which contains a primary component represented by the general formula of (1-n)(K1-x-yNaxLiy)m(Nb1-zTaz)O3-nM1M2O3 (where M1 is a divalent metal element, and M2 is a tetravalent metal element), and in Patent Document 2, a piezoelectric ceramic composition has been disclosed which contains a primary component represented by the general formula of (1-n)(K1-x-yNaxLiy)m(Nb1-zTaz)O3-nM1M2M3O3 (where M1 is a trivalent metal element, M2 is a monovalent metal element, and M3 is a tetravalent metal element). In addition, in both Patent Documents 1 and 2, x, y, z, m, and n are adjusted so as to satisfy 0.1≦x, y≦0.3, x+y<0.75, 0≦z≦0.3, 0.98≦m≦1.0, and 0<n<0.1.
According to Patent Documents 1 and 2, when a predetermined number of moles of a perovskite composite oxide M1M2O3 or M1M2M3O3 (such as BaTiO3, CaTiO3, or (Na0.5Bi0.5)TiO3) is solid-dissolved as a third component in (K,Na,Li) (Nb,Ta)O3, a piezoelectric ceramic composition is obtained which has a relative dielectric constant ∈r (=∈T/∈0; ∈T represents the absolute dielectric constant, and ∈0 represents the vacuum dielectric constant) of 1,000 or more, an electromechanical coupling factor kp of 25% or more, and a Curie point Tc of more than 200° C.
In addition, in Patent Document 3, a piezoelectric ceramic composition has been disclose which contains 0.005 to 0.15 moles of at least one metal element selected from the group consisting of Ag, Al, Au, B, Ba, Bi, Ca, Ce, Co, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, In, Ir, La, Lu, Mg, Mn, Nd, Ni, Pd, Pr, Pt, Rb, Re, Ru, Sc, Si, Sm, Sn, Sr, Tb, Ti, Tm, V, Y, Yb, Zn, and Zr with respect to 1 mole of a primary component represented by {Lix(K1-yNay)1-x}(Nb1-z-wTazSbw)O3 (where 0≦x≦0.2, 0≦y≦1, 0<z≦0.4, and 0<w≦0.2) and which has an open porosity of 0.4 percent by volume or less.
According to Patent Document 3, it has been disclosed that when the open porosity (the amount of dimples generated in the surface of the piezoelectric ceramic composition is represented by volume percent) is controlled to be 0.4 percent by volume or less by addition of the above Ag to Zr metal elements, the mechanical strength can be improved. In addition, since the composition represented by the general formula of {Lix(K1-yNay)1-x}(Nb1-z-wTazSbw)O3 is the primary component, by using superior piezoelectric d constant and electromechanical coupling factor kp of the compound represented by the above general formula, a piezoelectric ceramic composition having superior properties described above can be obtained.
In Patent Document 4, a piezoelectric ceramic composition has been disclosed which is represented by {(K1-xNax)1-yAgy}NbO3-z[Mα+][O2−]α/2 (where 0≦x≦1, 0≦y≦0.1, 0≦z≦0.05, and 0<y+z; Mn is at least one metal element selected from the group consisting of Mn, Mg, In, Si, Ga, and Sb; and α is an average valance of metal element M).
According to this Patent Document 4, by addition of predetermined amounts Ag and at least one metal element selected from the group consisting of Mn, Mg, In, Si, Ga, and Sb to (K,Na)NbO3, besides a decrease in the dielectric loss, that is, tan δ, and improvement in reliability, the piezoelectric d constant can be improved.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-228227
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-228228
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-244300
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-68835
According to Patent Documents 1 and 2, a high relative dielectric constant ∈r of 1,000 or more is obtained by addition of M1M2O3 or M1M2M3O3 as a third component to (K,Na,Li)(Nb,Ta)O3; however, when the content of the above third component is increased, since the electromechanical coupling factor kp is decreased, although an increase in piezoelectric d constant is slightly observed, the increase is not yet sufficient.
That is, among the piezoelectric d constant, the dielectric constant ∈T, and the electromechanical coupling factor kp, the following equation (1) holds.
[Equation 1]
                    d        =                  kp          ⁢                                                    ɛ                T                            Y                                                          (        1        )            
In the above equation, Y indicates Young's modulus.
Accordingly, in order to obtain a high piezoelectric d constant, the relative dielectric constant ∈r and the electromechanical coupling factor kp are both preferably increased; however, as described in Patent Documents 1 and 2, when only M1M2O3 or M3M4M2O3 is simply added as a third component to (K,Na,Li)(Nb,Ta)O3, the electromechanical coupling factor kp is decreased as the content of the third component is increased, and even if the relative dielectric constant ∈r can be increased, there has been a problem in that a desired sufficiently high piezoelectric d constant cannot be obtained.
In addition, according to Patent Document 3, the open porosity can be suppressed to 0.4 percent by volume or less by addition of a metal element, such as In, to {Lix(K1-yNay)1-x} (Nb1-z-wTazSbw)O3; however, according to an experiment carried out by the inventors of the present invention, a significant improvement in piezoelectric d constant could not be observed, and it was found that a piezoelectric ceramic composition having a desired high piezoelectric d constant cannot be obtained.
In addition, according to Patent Document 4, by addition of Ag, and In or the like to (K,Na)NbO3, d31 is improved; however, the improving rate is small, and it was found that a piezoelectric ceramic composition having a sufficiently high piezoelectric d constant cannot be obtained.
In addition, in recent years, concomitant with development of a technique to decrease a ceramic layer thickness, a multilayer piezoelectric ceramic electronic component driven by a high electric field has been developed and has been practically used.
In addition, as a piezoelectric material for a multilayer piezoelectric ceramic electronic component driven by a high electric field, a material having a high piezoelectric d constant at a high electric field, which is an electric field to be actually used, is preferable.
However, in general, a piezoelectric d constant at a high electric field, that is, at a practical electric field, and a piezoelectric d constant at a very low electric field, which is usually measured, are different from each other. Although the piezoelectric d constant at a very low electric field is high, the piezoelectric d constant at a high electric field is not always high.
That is, a piezoelectric material is composed of many regions, which are called domains, having different spontaneous polarization directions. In addition, at a very low electric field, only 180° domains respond which have a spontaneous polarization direction parallel to the direction of an applied electric field. On the other hand, at a high electric field, besides the response of the 1800 domains, since 90° domains, which have a spontaneous polarization direction perpendicular to the applied electric field direction, rotate toward the direction of the applied electric field, so that a large strain is generated; hence, as a result, a piezoelectric d constant which is higher than that at a very low electric field can be obtained. However, at a high electric field which is more than a certain predetermined electric field, since most of the 90° domains fully rotate and become 180° domains, a large displacement cannot be obtained. In addition, since the domain structure may vary with a material composition, although a material having a high piezoelectric d constant at a very low electric field is used, by the influence of the domain structure, the piezoelectric d constant at a high electric field may not be so much higher than expected in some cases.